This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a new negative electrode design having an anode active material sandwiched between two current collectors with an anode material capable of intercalating and de-intercalating the anode active material in contact with the opposite sides of the current collectors. The present negative electrode design is useful for high discharge rate secondary cells, such as those powering an implantable medical device.
Secondary electrochemical cells are typically built in a discharged state having a negative electrode of a carbonaceous material and a positive electrode of a lithiated material. A reverse potential is then applied to the cell to cause lithium ions at the positive electrode to migrate through the electrolyte and intercalate into the carbonaceous anode material. The cell is then discharged in a normal manner.
Graphitic forms of carbon are generally preferred for the anode material because they exhibit acceptable cycling capability and good reversible capacity, especially during initial change and discharge cycles. While graphite is commonly used, there are other carbonaceous materials which have better reversible capacities. One in particular is hard carbon. This material has excellent cycling characteristics, but suffers from a relatively large initial irreversible capacity.
Therefore, according to the present invention, a secondary cell is constructed having a xe2x80x9csacrificialxe2x80x9d piece of alkali metal as a portion of the negative electrode along with an alkali metal intercalation and de-intercalation material, such as a carbonaceous anode material. In the case of hard carbon, the sacrificial alkali metal is preferably a piece of lithium metal and is sized to compensate for the initial irreversible capacity of this anode material. After the cell is activated with an electrolyte, the lithium metal automatically intercalates into the hard carbon anode material. That way, the sacrificial lithium metal compensates for the generally unacceptable irreversible capacity of hard carbon. The superior cycling longevity of hard carbon now provides a secondary cell of extended use beyond that known for conventional secondary cells having only graphitic anode materials.
Accordingly, one object of the present invention is to improve the cycling performance of lithium ion cells by providing a new concept in negative electrode design. Further objects of this invention include providing a cell design for improving the capacity and utilization efficiency of lithium-containing secondary cells.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.